1. Field of the Invention
This invention relates to certain novel modified phenolic imide resins. More particularly, this invention relates to such resins which have improved properties.
2. Prior Art
Phenolic resins are a class of synthetic materials that have grow continuously in terms of volume and applications for over several decades. The building blocks used in greatest volume are phenol and formaldehyde. Other important phenolic starting materials are the alkyl-substituted phenols, including cresols, xylenols, p-tert-butyl-phenol, p-phenylphenol, and nonylphenol. Diphenols, eg, resorcinol (1,3-benezenediol) and bisphenol-A [bis-A or 2,2-bis(4-hydroxylphenyl)propane], are employed in smaller quantities for applications requiring special properties. In addition to formaldehyde, acetaldehyde or furfuraldehyde sometimes are employed but in much smaller quantities. The greater latitude in molecular structure, which is provided by varying the raw materials, chemistry, and manufacturing process, has made possible an extremely large number of applications for these products as a results of the array of physical propeties that arise from the synthetic options.
The early investigation of the reaction of phenol and formaldehyde began with the work of von Baeyer and others in the early 1870s as an extension of phenolbased dye chemistry. The initial experiments resulted in soluble, amorphous products whose properties elicited little interest. Insoluble, cross-linked products were reported in the late 1880s, but these products also were not perceived as useful materials. In 1899, the first patent for a phenolic-resin product intended for use as a hard-rubber substitute was granted. The first commercial product was introduced as a shellac substitute by the Louis Bluner Company in the early 1900's. Process patents were issued in 1894 and 1895 for ortho- and para-methylolphenol, respectively.
Key innovations in early phenolic-resin manufacture included control of the molecular structure and the use of heat and pressure to achieve desirable physical properties in filled compositions. Studies in the use of acidic or basic catalysts and of changes in the molar ration of formaldehyde to phenol resulted int he definition of two clases of polymeric materials which are refered to as Bakelite resins. Caustic-catalyzed products, which are prepared with greater than a 1:1 mol ratio of formaldehyde to pheno, can be used to form cross-linked, insoluble, and infusible compositions in a controlled fashion. With less than a 1:1 mol ratio of formaldehyde to phenol, the resultant products remain soluble; furthermore, acid catalysis yields permanently stable compositions, whereas base-catalyzed materials can be advanced in molecular weight and viscosity. Possibly of greatest importance to early commercialization, however, was the reduction to practice of the use of heat and pressure to produce essentially void-free molding compositions.
Resole resins are made with an alkaline catalyst and a molar excess of formaldehyde. Novolak or novolac resins are prepared with an acid catalyst and less than one mole of formaldehyde per mole of phenol. The initial reaction involved in the preparation of resolated novolaks is carried out with an acid catalyst and less than a 1:1 mol ratio of formaldehyde to phenol. After formation of the novolak, the pH is adjusted so that the reaction mixture is basis and additional formaldehyde is added. Resoles and resolated novolaks are inherently thermosetting and require no curing agent for advancement. Novolaks, by comparison, are thermoplastic and require the addition of a curing agent, the most commong being either hexamethylenetetramine or a resole. The stages of molecular weight advancement are characterized by liquid or solid phenolic polymer which is soluble in certain organic solvents and is fusible; a solid resin which is insoluble but swelled by organic solvents and, although softened by heat, exhibits essentially no flow; and an insoluble, infusible product which is not swelled by solvents nor softened by heat, ie, the system is in a highly cross-linked state.
Phenolic resins have many uses. For example, such materials are used as bonding agents in friction materials such as brake linings, clutch facings, transmission bonds and the like. For example U.S. Pat. Nos. 4,096,108; 4,268,657; 4,218,361; 4,219,453; and 3,966,670 describe various friction materials in which a phenolic resin is employed as the bonding agent. Phenolics are also used as molding materials, and as coatings and adhesives. Phenolics resins developed for nonflammability and long term temperature stability to 230.degree. C. have been studied in carbon-fiber composites. Potential for such composities lies in advanced aircraft application.
While present day phenolics exhibit several beneficial properties they suffer from a number of disadvantages which restrict their utility. For example, such materials exhibit less than desirable thermal oxidative stability. Other major problems of present day phenolic technology include a need for auxiliary chemicals such as hexamethylene tetraamine to crosslink the phenolic which often results in the production of volatile by-products such as ammonia during crosslinking. Still other problems result from the fact that crosslinking is often extensive and is not controllable.
Various modifications to phenolics have been proposed to obviate certain of the disadvantages attendant to these resins. For example, epichlorohydrin has been reacted with the hydroxyl groups of novalak forming epoxy novalak. Moreover, n-chloro-2-propene has been reacted with the hydroxyl groups of novalak to form the corresponding form methylon resin. Similarly, Japanese patent publication Nos. 59-149918 and 58-34822 describe a method of preparing a phenolic resin containing cyanate groups. In this method, a trialkyl ammonium salt of a phenol novolak is reacted with excess cyanhalogenide in an organic solvent.
Polyesterimides are known polymeric composition compounds. For example, polyesterimides and processes for their preparation are described in Great Britain Pat. Nos. 973,377; 1.070.364; 1,026,032; and 1,095,663; U.S. Pat. No. 3,839,264; D. F. Loncrini et al., J. Polym. Sci, Vol. 4, p. 440 (1966), and S. Das et al., J. Appl. Polym. Sci., Vol. 26, p 957 (1980).
Crosslinked polymers, for example, polycyanates (crosslinked polymers) derived by the polycyclotrimerization of aromatic cyanates are known. See for example, U.S. Patent No. 4,026,913, which describes cyanic acid esters of aromatic polycarbonates which can be cured to produce crosslinked polycyanurates. Also see the references, Kunst-stoffe, Bd., 58, pp. 827-832 (1968) by R. Kubens et al., and Dokl Akad. Nauk SSSR, Vol. 202, pp. 347-350 (1972) by V. V. Korsak et al., which describe the cyclotrimerization of aryl cuanurates and properties of crosslinked polymers derived therefrom. In addition, the referneces, U.S. Pat. No. 4,040,796 (1977) and German Offenlegungschrifte Nos. 2,549,529; 2,546,290; and 2,541,315 describe processes for producing certain poly functional cyanic acid esters, and cured products derived therefrom.
U.S. Pat. No. 4,157,360 describes cured compositions consisting essentially of a cross-linked cyanurate polymer and a thermoplastic polymer of at least film forming molecular weight. This composition possesses a Vical softening temperature of at least about 10.degree. C. above that of the thermoplastic polymer alone as determined by ASTM 1525; and an elongation-to-break value which is at least twice as great as that of the crosslinked polymer alone or determined by ASTM D-638 at room temperature.